docs: add missing parameter to code snippet (#26)

README.md

@@ -91,6 +91,10 @@ import com.openai.models.ChatCompletionCreateParams;
 import java.util.List;
 
 ChatCompletionCreateParams params = ChatCompletionCreateParams.builder()
+    .message(List.of(ChatCompletionMessageParam.ofChatCompletionUserMessageParam(ChatCompletionUserMessageParam.builder()
+        .content(ChatCompletionUserMessageParam.Content.ofTextContent("Say this is a test"))
+        .role(ChatCompletionUserMessageParam.Role.user)
+        .build())))
     .model("gpt-4o")
     .build();
 ChatCompletion chatCompletion = client.chat().completions().create(params);